In conventional coffee brewing machines, water stored in a reservoir is heated by means of an electrical resistance to a temperature just below boiling and is operable, in addition to heating the water, to circulate the water from the reservoir to a discharge point above the coffee grounds maintained in a filter element. The hot water flows through the coffee grounds and filter into a coffee pot supported below the filter. Coffee brewing machines presently on the market are based on that arrangement and generally have their reservoir provided with graduations according to the number of cups (2, 4, 6, 8 . . . ) desired to be brewed. Each time the coffee brewing machines are used they operate to exhaust the water contained in the reservoir, which causes noise because of the use of check valves which fill with air, causing water vapor to be formed.
Another problem with such machines occurs when more water than needed for the number of cups to be brewed has been poured into the water reservoir; the user must use unconventional means to remove the excess, such as drawing down clear water into the coffee pot, first removing the filter and coffee grounds, or introducing a small cup to draw water before the coffee and the filter are placed in position in order to obtain the desired amount of water in the reservoir to begin the brewing process.
Moreover, to avoid build-up of excessive scale and for good operation purposes, it is recommended to leave water in the reservoir, heating element housing and tubing when the coffee brewing machine is not in operation. This is, however, not very practical with coffee brewing machines of the conventional type because the reservoir has been emptied completely, and the coffee pot cannot be used to pour water into the reservoir because it contains brewed coffee; thus it is difficult for the user to put water in the reservoir again.